4 mana too heavy for this affect? A counterspell and essentially a time walk in Vintage, stopping combo, Mentor's, Scab-clans, maybe even Thalia? Not quite sure with the wording, ALL is a pretty clutch term. Thoughts?

Yes...4 mana is much greater than 0 when comparing this to Mindbreak Trap.

It's also not as good as you want it to be - a key part of why Flusterstorm and Mindbreak Trap are so good is that it's relatively easy to get a 2 for 1 or more with them, by casting an instant in response to their spell, then countering both their counterspell and the original spell. This will exile your own spell in that situation... you can't even use it in a counter battle as it will exile the spell you were trying to resolve.

This card is unplayable in the Eternal Formats. It's actually a worse Voidslime.

"One key to the continued health of Magic is diversity. It is vitally important to ensure that there are multiple competitive decks for the tournament player to choose from."

It might have legacy use, but I doubt vintage. It's templated specifically to provide a complete answer to large Eldrazi with cast triggers. It exiles without countering, so it'll hit uncounterable spells like big Emrakul, and will stifle that trigger. There's also some splash usefulness against Bloodbraid and Shardless.

In vintage, the best case scenario is probably countering a gush, and devouring all of the mentor triggers with it. No new token, and existing ones won't pump. Something tells me the gush deck will just keep trucking and find another spell to cast.

It's an interesting design that's aimed at relevance in competitive play. Unfortunately, it needs a lot more to make the cut in Vintage, despite the fact that resolving it does interact with many things in the format. Discretion in which abilities/spells are countered would be a start, ie, "exile any number of spells and/or abilities" and then it would need some combination of an alternate casting cost, an outright lower casting cost, and auto-protection for itself on the stack, ie "cannot be countered" or something similar. By costing too much and being wide open to all opposing countermagic right down to the lowly Pyroblast, the card fails.

Samantha: “Matt, the deck lost to Merfolk.”
Matt: “I don’t build decks to beat Merfolk. In any format.”

@MaximumCDawg Probably. It's a clean answer to Emrakul, which is pretty much the only saving race of the card. Unfortunately Emrakul is rarely cast in legacy, so it's moot.

But how often will you be able to counter Emrakul, yet not counter the Show and Tell that put Omniscience into play? And if you have your pick of which spell to counter, clearly you will want to counter Show and Tell.

This card is completely unplayable in all Eternal formats, thanks to its high casting cost and vulnerability to cheaper, better counterspells like Flusterstorm and Pyroblast, as Brian points out. Who knows if it will see play in Standard? I don't think that Standard decks can afford to sit on four mana to keep up Summary Dismissal, unless a lot of support cards for a control strategy remain to be spoiled.

@evouga I completely agree. I'm just pointing out that it does provide an answer that no other card can replicate. Nothing else clears the stack so completely, so it's nice to see a genuinely new counter even if they have costed it out of playability.